GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE LATE ARCHEAN SKJOLDUNGENALKALINE IGNEOUS PROVINCE, SE GREENLAND

The present work reports the first broad geochemical investigation of the recently discovered late Archean (2700 Ma) Skjoldungen alkaline ig neous province (SAP) in southeast Greenland. The rocks studied range i n composition from ultramafic to felsic and comprise pyroxenites, horn blendites, hornblende norites and diorites, monzonites, syenites, and nephelinitic rocks and carbonatites. Various lithologic units from the host Archean gneissic basement are also investigated. The magmatic ro cks show remarkably coherent major element, trace element, rare earth element (REE) and Sr and Nd isotope Systematics, suggesting a petrogen etic relationship. The most important geochemical features are high no rmative proportions of nepheline, forsterite and albite, low TiO2 (<1. 5 wt %) and moderate FeO (total) (<12 wt %) contents, enrichments in l arge ion lithophile elements (LILE) and light rare earth elements both absolute and relative to high field strength elements (HFSE) that dis play large negative anomalies, and generally low to moderate abundance s of compatible elements. Field relations and REE and compatible eleme nt systematics among Skjoldungen rocks suggest that mafic and ultramaf ic hornblende-rich samples may represent cumulate lithologies of the r egional parental magma. On the basis of mineral data, this is deduced to have had mg-number of 0.64, shoshonitic affinities (K2O similar to 1.5 wt %), been close to silica saturation and volatile rich. Major el ement, trace element and REE Systematics further suggest that felsic i ntrusions are related to the mafic regional parental magma through ext ensive olivine, hypersthene and hornblende fractionation. Lack of corr elation between La/Yb and other critical trace and REE ratios indicate s that apatite, zircon and titaniferous minerals were not important cu mulus phases at advanced stages of evolution. The measured Sm-Nd whole -rock isochron age is 2716 +/- 23 Ma (2 sigma error) [mean square of w eighted deviates (MSWD) = 1.4]; whereas linear regression of the Sr is otope data yields an age of 2604 +/- 7 Ma (2 sigma error) (MSWD = 22.2 ). The age obtained by Nd isotopes is corroborated by U-PB zircon resu lts (2698 +/- 7 Ma), suggesting that the Sm-Nd system remained closed since crystallization. By contrast, the 100 Ma younger age obtained by Sr isotopes suggests that the Rb-Sr system has been disturbed Initial Nd-143/Nd-144 ratios span a narrow range corresponding to E(Nd)(2.7 G a) = +0.74 to -1.09, whereas initial E(Sr) values at 2.7 Ga cover a co mparatively larger interval from -10 to +20. Neither initial E(Nd) nor initial E(Sr) values conform to previously suggested mantle depletion curves and no meaningful correlation exists between Nd and Sr isotope s for the Skjoldungen magmatic rocks as a whole. Although compositiona lly, heterogeneous, the analyzed suite of samples from the host agmati tic basement is extremely homogeneous with respect to age, with T-CHUR crustal residence times around 2700-2800 Ma confirming limited availa ble isotopic evidence. Large-scale assimilation of Archean crust or re cycling of sediments derived from the local basement into the mantle s ource fails to explain adequately negative Nb anomalies and low E(Nd) signatures characteristic of the Skjoldungen intrusions. Rather, the n ear-chondritic isotopic composition of Nd in the Skjoldungen samples t ogether with the decoupled LILE and HFSE enrichment and slightly posit ive E(Sr) values are considered to reflect characteristics of the mant le source in a subduction zone environment. The geodynamic site hostin g the Skjoldungen province thus may be an early manifestation of moder n-style plate tectonics.